Automatic fire detection for a microwave oven

Abstract

A method and apparatus for anticipating the occurrence of a fire in the cooking cavity of a microwave oven. The oven is provided with a blower for continuously circulating outside air through the cooking cavity to carry away gases by food loads being heated therein. A sensor responsive to the concentration of gases in the circulating air monitors the gas concentration level and generates an output signal representative thereof. Sensor monitoring circuitry responsive to the sensor output signal is operative to determine the rate of change of the gas concentration level, and a detect a characteristic rate of change indicative of the load being heated closely approaching its combustion point. Upon detection of the characteristic rate of change, power to the oven is turned off and and indication is provided to the user that a combustion condition in the oven cavity is imminent.

What is claimed is: 
     
       1. A method of anticipating a fire condition in the cooking cavity of a microwave oven of the type having means for continuously circulating air through the cavity to remove gaseous emissions from the food being heated in the cavity and sensor means for sensing the concentration of such gases in the circulating air, said method comprising the steps of: circulating air through the cavity to remove therefrom gases emitted from the food load being heated in the cavity;   sensing the concentration level of such gases in the circulating air;   periodically determining the rate of change of concentration level;   monitoring the rate of change to detect a rate of change characteristic of the temperature of the food load in the cavity closely approaching its combustion temperature including the steps of detecting a first relatively slow rate of change indicative of the load having been cooked to its drying out stage and thereafter detecting a second relatively rapid rate of change, detection of said second rate following said first rate being indicative of closely approaching the combustion temperature of the load; and having detected said second rate of change, turning off power to the oven.   
     
     
       2. A method of anticipating a fire condition in the cooking cavity of a microwave oven of the type having means for continuously circulating air through the cavity to remove gaseous emissions from the food being heated in the cavity and sensor means for sensing the concentration of such gases in the circulating air, said method comprising the steps of: continuously circulating air through the cooking cavity;   sensing the concentration level of gases emitted by the food in the circulating air;   detecting a concentration level above a predetermined reference level indicative of having heated the food sufficiently to cause the food to emit gases into the circulating air;   having detected a concentration level greater than the reference level, thereafter periodically determining the rate of change of the concentration level as a function of time;   monitoring the rate of change of the concentration level to detect a relatively small rate of change characteristic of the drying out of the food load;   having detected the small rate of change, thereafter monitoring the rate of change to detect a relatively large rate of change; and   upon detection of the relatively large rate of change, turning off power to the oven and providing an indication to the user that a combustion condition in the oven is imminent.   
     
     
       3. A method of anticipating and preventing a fire condition in the cooking cavity of a microwave oven of the type having means for circulating air through the cavity and sensor means for sensing the concentration of gases in the circulating air emitted from food loads being heated in the cavity, said method comprising the steps of: (a) continuously circulating air through the cooking cavity of the oven during the cooking cycle;   (b) periodically sensing the concentration level in the circulating air of gases emitted from the food during cooking and generating a voltage signal representative of the sensed concentration level;   (c) monitoring the voltage signal to detect a voltage level indicative of having completed an initial portion of the normal cooking cycle;   (d) having detected an indicative voltage, thereafter periodically computing the rate of change of voltage with time;   (e) monitoring the rate of change of voltage to detect a small rate of change of voltage characteristic of the drying out of the food load;   (f) having detected the small rate of change, thereafter monitoring the rate of change to detect a large rate of change characteristic of a relatively rapid increase in the concentration of gases in the circulating air; and   (g) upon detection of the large rate turning off the power to the oven and providing an indication to the user that a combustion condition in the oven is imminent.   
     
     
       4. A method for anticipating the occurrence of a fire in the cooking cavity of a microwave oven of the type having means for circulating air through the cavity during the cooking process and a sensor for determining the concentration of gases in the circulating air emitted by foods being cooked in the cavity, which concentration level as a function of time is divided into three phases: a normal cooking phase characterized by an initial brief period of relatively constant concentration level followed by a period of relatively rapidly increasing concentration level; a drying out phase characterized by a relatively slowly increasing concentration level, and a pre-combustion phase, characterized by a relatively rapidly increasing concentration level, said method comprising the steps of: circulating air through the cooking cavity of the oven;   detecting a concentration level of gases in the circulating air indicative of the cooking cycle having progressed beyond the initial period of the normal cooking phase for the food being heated;   thereafter periodically determining the rate of change of concentration level as a function of time;   detecting a first rate of change characteristic of the drying out phase;   having detected said first rate of change, thereafter detecting a second rate of change indicative of the pre-combustion phase; and   upon detection of said second rate of change, turning off power to the oven and providing an indication to the user that a fire condition in the cavity is imminent.   
     
     
       5. A control arrangement for a microwave oven comprising: means for generating microwave energy for heating foods received in the cooking cavity;   means for continuously circulating air through the cooking cavity to remove therefrom gases emitted by the food load being heated in the cavity;   sensing means for sensing the concentration level of such gases in the circulating air and generating a first signal having a magnitude representative of the sensed concentration level;   sensor monitoring means responsive to said first signal and including means operative to periodically determine the rate of change of concentration level as a function of time, and means operative to detect a rate of change indicative of closely approaching the combustion point of the load being heated in the oven, said detecting means including means for detecting the occurrence of the second of two periods of relatively rapidly increasing concentration level separated by a period of relatively slowly increasing concentration level, and generating a second signal upon detection of said second period; and   means for terminating operation of said microwave energy generating means and said air circulating means in response to said second signal.   
     
     
       6. A control arrangement for a microwave oven comprising: means for generating microwave energy for heating foods received in the cooking cavity;   means for continuously circulating air through the cooking cavity to remove therefrom gases emitted by the food load being heated in the cavity;   sensing means for sensing the concentration level of such gases in the circulating air and generating a first signal having a magnitude representative of the sensed concentration level;   sensor monitoring means responsive to said first signal and including means operative to periodically determine the rate of change of concentration level as a function of time, and means operative to detect a rate of change indicative of closely approaching the combustion point of the load being heated in the oven including means for detecting a relatively slow rate of increase in concentration level indicative of the food load being heated to its drying out stage and thereafter detecting a relatively rapid increase in concentration level indicative of closely approaching the combustion point of the food load and generating a second signal upon detection of said relatively rapid increase; and   means for terminating operation of said microwave energy generating means and said air circulating means in response to said second signal.   
     
     
       7. A control arrangement for a microwave oven which anticipates a fire condition in the cooking cavity of the oven, said control arrangement comprising: means for generating microwave energy for heating foods received in the cooking cavity;   means for continuously circulating air through the cooking cavity to remove therefrom gases emitted from the food load being heated in the cavity;   sensing means for sensing the gas concentration level of the circulating air and generating a first signal having a magnitude representative of the sensed gas concentration level;   sensor monitoring means responsive to said first signal including: means for detecting a signal magnitude indicative of a gas concentration level greater than a reference level; means for determining the rate of change of said signal magnitude; means for first detecting a relatively slow rate of change following detection of said concentration level greater than said reference level and thereafter detecting a relatively rapid rate of change;   means responsive to said sensor monitoring means operative to de-energize said microwave energy source and to terminate operation of said air circulating means following detection of said rapid rate of change; and   means responsive to said sensor monitoring means operative to provide an indication to the user that a fire condition in the cavity is imminent following detection of said rapid rate of change.